Keep Tahoe Blue: Water Clarity Improves Despite Extreme Weather

Despite a wet, weather-extreme 2011, the clarity of Lake Tahoe improved, keeping pace with a decade-long improvement trend, according to an annual report from the Tahoe Environmental Research Center at UC Davis.

Yet the underlying trends reveal a more complex picture of the famed lake's ecosystem, according to the “Tahoe: State of the Lake Report 2012.” While the lake's celebrated blue waters has long been the most visible, widely used indication of its health, many environmental and water quality factors are at play, including clarity, algae, invasive species and the effects of climate change on precipitation, lake temperature and density stratification, according to a news release.

Overall, 2011 was an “unusual” year for Lake Tahoe, in that the weather was extreme – the 2010-2011 winter was one of the coldest and wettest on record – and resulted in more precipitation than usual in the form of snow and spring snowmelt that arrived releatively late. “Despite the cold winter and cool July, the annual average surface water temperatures rose by 0.6 F,” the release stated.

Residents from San Carlos and surrounding cities help make Lake Tahoe one of the most popular places to visit.

The data reveal how natural forces and human activity affected the lake's clarity, physicics, chemistry and biology since the university began continuous montitoring of the lake in 1968. UC Davis Tahoe Environmental Research Center Director Geoffrey Schladow said 2011 “has defied conventional wisdom in many ways.”

“In the past, very wet years have led to decreases in lake clarity, wheras we are now seeing the opposite,” he stated in the release. “This only reinforces the fact that the underlying driving forces are themselves starting to change.”

Researchers found that the length of time a summerlike stratification – where layers of water form with different temperatures – persists has increased by almost 20 days during the past 43 years, a likely outcome of climate change, the release stated. “Researchers fear that if this trend continues, oxygen replenishment to the bottom of the lake will become less frequent,” the release stated.

Also, the annual average lake clarity improved significantly compared to 2010, increasing by 4.5 feet. However, year-to-year fluctuations are normal, which is why TERC researchers emphasize that long-term trends are a better indicator of lake health.

According to the UC Davis News & Information:

Stability, defined as the energy needed for mixing layers of lake water, was calculated for the first time this year.

The 2011 winter clarity level of 84.9 feet was in keeping with a decade-long trend of actual improvement. The report speculates that the improved winter clarity of 11.9 feet over 2010 may be due to recent efforts to reduce urban stormwater flows to the lake, though researchers emphasize the need for a monitoring program to substantiate that idea. Meanwhile, summer clarity of 51.5 feet in 2011 was the second worst on record.

A potential culprit to reduced summer clarity is a microscopic algae cell called Cyclotella. The tiny cells have grown exponentially in the past five years, scattering light and reducing clarity. Research shows that times of the highest concentrations of Cyclotella coincide with the lowest summer clarity levels.

Clarity is measured by the depth at which a 10-inch, white Secchi disk remains visible when lowered beneath the water's surface. The measurements have been taken since 1968, when the Secchi disk could be seen down to 102.4 feet.

Newly included this year is a summary of recent, ongoing research:

* The study said that the 2007 Angora Fire, which burned 3,100 acres, or 9 percent of the Upper Truckee River drainage, has had almost no effect on lake water quality.

* An experiment using rubber mats on a half-acre site in the southeast portion of Lake Tahoe to control the spread of Asian clams appears to be effective. In June 2010, researchers placed mats over Asian clam infestations to smother them. When the barriers were removed four months later, more than 98 percent of the clams had been killed. A year later, clam density was still reduced by more than 90 percent, the report said. The technique is currently being modified to control Asian clams in Emerald Bay.

* Researchers also observed the effects of pathogens, insects and mortality on forest health and found that trees in the upper montane (dominated by red fir and western white pine) elevations experienced the highest levels of mortality. The lowest levels of tree death were in subalpine forests. The most common forms of forest pests are bark beetles and dwarf mistletoes. The exotic and invasive pathogen Cronartium ribicola, which causes white pine blister rust, also
threatens forest health.

The annual State of the Lake report informs non-scientists about the most important factors affecting lake health and helps influence decisions about ecosystem restoration and management within the Lake Tahoe Basin.

"Lake Tahoe can serve as an example to many other places in the nation," Schladow said. "Science is being used to guide management of this precious resource, to inform honest debate on the restoration challenges, and to quantify the impact of the investments that have been made to date."

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